U.S. Pat. No. 4,323,530 describes a process for compressing granulates to form tablets, coated tablet cores, and the like wherein before each compression process a certain amount of lubricant in liquid or suspended form is applied to the affected zones of the pressing tools by means of an intermittently operating nozzle system. This type of lubrication ensures that no lubricant, such as magnesium stearate, has to be added to the granulate which is to be compressed. This results, for example, in pharmaceutical compositions with a substantially better bioavailability of the active substance contained therein; moreover, significantly reduced quantities of lubricant are required. According to the process described in this patent, the lubricant is applied by means of directed spraying of specific zones of the pressing tools with the liquid or suspended lubricant by use of preferably single-substance or two-substance nozzles or dies. However, when these nozzles are used, and particularly when two-substance nozzles are used wherein air and lubricant liquid are delivered simultaneously, it has been found that droplets form with a particle spectrum which depends in its breadth upon the supply of air. These nozzles tend to produce an undesirable mist, which can lead to contamination of the tablet press, particularly the pressing plate.
The use of single-substance nozzles through which the liquid lubricant is sprayed intermittently onto the corresponding parts of the pressing tools just before each separate pressing operation has also demonstrated a tendency to contaminate the tablet-pressing plate due to the formation of a cone of spray or the occurence of stray drops of different diameters within the boundaries of the spray cone. However, when used in fast-operating tablet presses with actuating intervals of up to 5 msec., the single- and two-substance nozzles also fail to give a constant dissolution of the liquid lubricant, and they generate not only individual droplets but also sequences of drops consisting of drops having different diameters. The result is that there is no guarantee of a constant action over the intended zones of the pressing tools.
It has already been proposed (see German Offenlegungsschrift No. 29 32 069) that these disadvantages could be overcome by dotting the liquid or suspended lubricant, before each pressing operation, onto the affected zones of the pressing tools in defined quantities and in the form of discrete droplets of defined volume by means of a piezoelectric transducer in conjunction with corresponding nozzles in a directed manner. However, a certain disadvantage of this process is that the liquids to be sprayed are subject to stringent requirements with regard to their viscosity and surface tension. Only if certain limits are adhered to for the viscosity and surface tension is it possible to dot the liquids satisfactorily over the intended pressing zones. Moreover, this system is sensitive to dust and is not readily suitable for the lubrication of pressing tools for compressing powdery or non-granulated materials with a high powder content, such as, for example, sorbitol compositions in the food industry.